Print actuator arrangement and encoder

ABSTRACT

A type carrier presents characters serially to a print station which includes a recording medium. A signal controlled actuator at the print station is interposed in the print character path just before the desired character arrives and is struck on an angled cam surface by the character causing the character to strike the recording medium and forcing the actuator away from the character path. A binary encoder which issues signals determinative of what character is at the print station issues as many as 3/2 . 2n unique signals with n code tracks and n + 1 transducers.

United States Patent [191 Scott, Jr.

[451 Aug. 26, 1975 PRINT ACTUATOR ARRANGEMENT AND ENCODER [75] Inventor: Philip Romeo Scott, Jr., Berlin,

[30] Foreign Application Priority Data Mar. 29, 1972 United Kingdom 14683/72 Mar. 29, 1972 United Kingdom 14684/72 [52] US. Cl 197/53; 10l/93.19 [51] Int. Cl B4lj l/32 [58] Field of Search 197/53, 52, 18, 15, 6.6, 197/48, 55, 6.7, 54; 101/93 C, 110

[56] References Cited UNITED STATES PATENTS 2,902,132 9/1959 Walker 197/52 3,289,805 12/1966 Kleinschmidt et al. 197/53 X 3,315,775 4/1967 Schonfeld 197/15 3,355,001 11/1967 Reed et al 197/18 3,366,214 1/1968 Tutert et a] 197/48 44 SOURCE OF COMPARATOR CHARACTER CODES 42 ENCODER 3,406,807 10/1968 Sasaki et al 197/53 X 3,442,365 5/1969 Ragland et al.-.. 197/55 X 3,715,978 2/1973 Raider 197/53 X FOREIGN PATENTS OR APPLICATIONS 380,843 9/1932 United Kingdom 101/93 C 1,219,253 1/1971 United Kingdom 101/93 C Primary ExaminerEdgar S. Burr Assistant Examiner-R. T. Rader Attorney, Agent, or F irmRaymond E. Smiley; Edward J. Norton 4 [5 7] ABSTRACT A type carrier presents characters serially to a print station which includes a recording medium. A signal controlled actuator at the print station is interposed in the print character path just before the desired character arrives and is struck on an angled cam surface by the character causing the character to strike the recording medium and forcing the actuator away from the character path. A binary encoder which issues signals determinative of what character is at the print station issues as many as 3/2 2" unique signals with n code tracks and n 1 transducers.

6 Claims, 4 Drawing Figures Pmmnwsz m 3,901,371

SHEET 1 0F 2 40' SOURCE OF COMPARATOR CHARACTER 7 CODE PRINT ACTUATOR ARRANGEMENT AND ENCODER BACKGROUND OF THE INVENTION High speed mechanical printers both of the line-at-atime and character-at-a-time varieties commonly employ a rapidly moving type carrier for presenting a plurality of characters serially at one or more print stations. Ideally, when a desired character is passing a print station, a print hammer is tired to drive the character into the recording medium or vice versa. If the hammer is not fired at just the right time, the character struck is not printed at the desired print location. Improper firing may be caused by improper timing of the signal for the print hammer or by variations in mechanical parameters associated with the movement of the hammer to the print position.

If after being fired, the hammer does not retract promptly, smearing may occur. Smearing results when the moving character-remains in contact with the recording medium for too long a time interval.

US. Pat. No. 3,315,775 to A. Schonfeld suggests one solution to the first problem. There, in FIG. 1, a print wheel continuously turns about shaft 12. When a desired character is next to move into print position, a cam shaped interposer 32 moves vertically into a position of interference with studs 10. As shown in FIG. lb, as the next stud comes into contact with surface 54 of the interposer, the entire print wheel is moved into contact with ribbon 56 causing the desired character to be rolled against the ribbon at a time which is determined only by the location of stud 10. Then, after stud passes and before the next stud moves opposite interposer 32, the latter must be moved down out of the path of the studs. If the interposer 32 is not moved down out of the way rapidly, the next character will also be printed, an undesirable result.

While this system operates for its intended purpose, it appears to be suitable only for relatively slow printing speeds (a typewriter is contemplated rather than a high-speed printer). There must be sufficient time available between the passage of two adjacent studs 10, to permit the vertical upward movement and then the vertical downward retraction of the interposer 32. This means that the print wheel 5 must rotate relatively slowly.

In US. Pat. No. 3,353,482 issued to A. A. Sariti, the character elements 3, FIG. 2, are mounted on flexible members 16 so that during printing the character is actually stopped even though the character driving element 2 is moving. This solves the smearing problem discussed above. However, if the hammer 9 does not reach the record strip 10 at the right time, misregistration may occur.

Some type of encoder device is typically employed to provide a binary indication of the character on the type carrier next approaching the print station. It is often desired to make the encoder as light in weight and small in size as possible. One way to accomplish this is to use fewer binary code tracks (and in this way reduce the size and mass of the encoder) than the n normally required to produce 2'' 1 to 2" unique codes.

U.S. Pat. No. 3,229,280 to D. M. Chapin indicates how 2" unique codes may be produced with only n/2 code tracks. However, two heads are required per track and Chapins system is useful only for producing a cyclic or reflected binary code. It is not useful for the standard binary code or other special binary codes.

SUMMARY OF THE INVENTION A printer includes a plurality of symbol elements movable seriatim past a print station and an actuator means. The elements are normally biased away from said recording medium at least at said print station. The actuator means includes a cam member movable between two positions, the first with its cam surface positioned in the path taken by the symbol elements as they are moved past said print station and the second out of said path. The actuator is responsive to a command signal indicating that a desired symbol element is next approaching said print station for moving said cam member into said first position whereby the desired symbol element strikes the cam surface and moves into momentary contact with the recording medium and in response to the symbol element striking the cam surface, the cam member moves to said second position.

A code wheel useful with the printer includes n distinct sensing areas, each representing a different power of two and solely n l scanning elements, n being arranged to scan respectively the n sensing areas and the additional one being arranged to also scan the most significant area.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partly block and partly mechanical schematic diagram of a serial printer which includes the present invention;

FIG. 2a is a schematic showing of an encoder assembly useful in the printer of FIG. 1;

FIG. 2b is the same encoder in a different position;

FIGS. 3a through 3d illustrate the mechanical relationship between the actuator and type carrier wheel of FIG. 1 at different positions of the wheel; and

FIG. 4 shows a portion of an alternate form of type carrier wheel.

DETAILED DESCRIPTION In FIG. 1 a serial printer 10 includes a platen 12 about which a recording medium 14 is passed. An inked ribbon 16 extending between spools l8 and 20 is interposed between the recording medium 14 and a print wheel 22, the latter being shown partly in phantom. The movable printing member 22 may take many forms. For example, a plurality of symbol elements in the form of embossed characters, numerals and other symbols 24 are attached to a like number of resilient finger-like elements 26. The free end of elements 26 are movable toward and away from the recording medium. Associated with each symbol element 24 is a rearwardly projecting stem 28, the purpose of which will become evident shortly.

The print wheel 22 is attached to shaft 30 to be driven by a suitable means such as a motor 32. Also attached to shaft 30 is an encoder assembly 34 which converts the angular position of shaft 30 into a plurality of discrete binary encoded signals in a plurality of conductors 36, the number of code combinations corresponding to the number of symbol elements 24. Thus, if there are, for example, 96 symbol elements 24, encoder 34 must be capable of providing 96 unique combinations to indicate which of the characters is opposite a print station (to be described).

Encoder 34 is coupled to a comparator 40. A source of character codes is coupled to comparator 40 and supplies to the latter a multiple bit character code via conductors 42. This character code indicates the next character to be printed. The source, while not shown, is generally from a high speed device such as a digital computer or the like. The comparator, which is of conventional design applies an output signal to one shot 44 when the code present at conductors 42 is equal to that appearing on conductors 36. In response to this signal, one shot 44 produces a momentary pulse which is amplified by power amplifier 46 to provide momentary energizing power to an actuator means 50.

Actuator means 50 includes a solenoid 52 and a movable member 56 formed with a cam surface 58 at its free end. The cam member 56, which will be described in greater detail in connection with FIG. 3, is movable between a first position in which the end portion 58 is interposed in the path of travel of projecting stems 28 of print wheel 22 and a second position (the extreme limit of which is shown) retracted away from the interposed position. Stops 60 and 62 (the latter shown in phantom) provide the extreme limits between which cam member 56 may be positioned. The cam member is driven to the first position as a result of signals received from amplifier 46 and is driven to the second position by the interaction between end 58 of the cam member and projecting stems 28 in a manner to be described. A spring 64 having a relatively low spring constant is coupled between cam member 56 and a source of mechanical ground. The purpose of the spring 64 is to keep the cam member in the fully retracted position from which it might have a tendency to move due to vibration or other action such as gravity (if cam member 56 is tilted from the horizontal).

The end 58 of cam member 56 is located at an imaginary region relative to the printer called a print station. The print station is the region at which it is desired to print characters on recording medium 14. Since a serial printer is illustrated, there must be provided some means to move the recording medium laterally with respect to print wheel 22 and actuator means 50. Such a means, which is not shown, may either move the recording medium in the direction of the axis of platen 12 or the recording medium may remain stationary and the print wheel 22, motor 32 and actuator 50 may be moved parallel to the platen axis as one mechanical assembly.

FIG. 2 illustrates one type of encoder useful in the printer of FIG. 1. The encoder illustrated, by way of example, has three code bands and four pickup heads. While, in practice, a larger number of code bands (actually six for 96 symbols) and heads (seven for 96 symbols) may be employed for encoder 34, the principle of operation remains the same. Such an encoder is smaller in diameter and therefore lighter in weight than a conventional encoder with the normal number of bands. Alternately an encoder which is the same size asa conventional encoder of the disc type of FIG. 2 may be made more accurate than a conventional encoder by moving the bands farther from the center.

Binary encoders which provide up to 2" code combinations normally require n code bands. For example,

an encoder with three code bands is normally capable of providing eight codes and if one desired 12 codes the code wheel normally would have four code bands. The I encoder of FIG. 2 employs only three code bands labeled 92a, 94a and 960 respectively (and four heads 92b, 94b, 96b and 98) and is capable of providing 12 codes, as listed in Table I.

From FIG. 2a, it is seen that transducer 92b, reads the band having a value 2", 94b reads 2, 96b reads 2 and 98 reads 2 as shown in Table I. Note that in providing the binary code for the decimal numbers 0 to l l, the binary digit 2 and the binary digit 2 each has a pattern of eight zeros and four ones. The only difference is that the pattern for 2 occurs four segments later than that for 2 Thus, the same track can be used for the 2 and the 2 digits by placing transducers 98 and 96b on the same track with transducer 98 positioned four segments behind the position of transducer 96b.

In general, in the arrangement illustrated, n bands or sensing areas may be arranged to provide between 2" l and 3/2 2" unique code combinations as will be described. This is important in printer applications where the ASCII set of 96 printable characters (3/2 2") is often used. These sensing areas may be in the form of bands and may be configured in any one of a number of ways. For example, the bands may be concentric as shown or may be circumferential bands about a cylinder. Further, the various light areas in each band such as and dark areas such as 82 on a code wheel 84 may represent binary Os and ls respectively and may be in colors exhibiting first and second reflective areas or transparent and nontransparent areas or areas of nonconductive and conductive material or areas magnetically recorded in one polarity and in a second polarity. There is one more transducer or head than there are code bands regardless of the number of bands. Two of the transducers 98 and 96b are positioned over one band, the most significant band 96a, and the remaining two transducers 92b and 94b are located over bands 92a and 94a, respectively. The transducers are appropriate to the type of information encoded on code wheel 84 or light transmitted through the code wheel from its reverse side or they may be simply electrical contacts or they may be electromagnetic transducers.

The boundary lines 99 and numerals 0-11 are not present on an actual code wheel. The numerals merely illustrate the base 10 equivalent of the binary number that will be manifested on conductors 2 -2 when the portion of the code wheel associated with a numeral is opposite 92b96b. Thus, in FIG. 2a the conductors 22 all contain binary Os to represent the base 10 digit 0. In FIG; 2b the code wheel 84 has been rotated so that the binary equivalent, 1001, of the numeral 9 is present at the transducers. It should be noted that if segments 82 represent conduction paths then some means (not shown) must be utilized to provide potential to the various segments 82. 1

Each band may have a number of contiguous segments of one binary value equal 2" where n correspo ls to the power represented by a band. Thus, in FIG 1, the inner band, the 2" band, has every other segment representative of binary l, and every other segment representative of binary O. The outer band, the 2 band, has four contiguous segments of one binary value followed by four contiguous segments of the other binary value. It should also be obvious that the three transducers 92b-96b need not be in line as illustrated, but may be offset so long as the associated bands are equally offset.

The relative positions of transducers 98 and 96b are somewhat critical. If it is desired to produce the standard binary code as reproduced in Table I, which can only be done for exactly 3/2 2" unique codes, then element 98 must trail element 96b by a distance equal to the arcuate length of the binary 1 indicating portion of the most significant band. Thus, if the black portion of band 96a represents a binary 1, since it is four positions long or 4/12 of the entire circle, then transducers 96b and 98 mustbe located 4/12 of a circle apart. However, if the only requirement is that there be 12 unique combinations, then the two transducers 90 and 92 need be spaced apart, at minimum, the distance occupied by the portion of the most significant band which represents a l or, in the example given, at least 4/12 of the circle away from the other transducer either clockwise or counterclockwise.

The principle just described is applicable to any number of bands. Thus, while seven bands would normally be required to produce 96 unique combinations (the same seven bands would also be able to produce as, many as 128 unique combinations), in the present invention only six bands are required, each associated with a separate transducer. An additional transducer is positioned opposite the most significant band and at a distance from the other transducer as set forth in the rules above.

It is also possible to provide an encoder which produces 2" I through 3/2 2' 1 unique codes using n bands and n l transducers but it will not be possible to produce the standard binary code. The second transducer, associated with the most significant band, is located in the two segments which, in an encoder having 3/2 2" segments, is one-third of the distance around the encoder from the other transducer. Thus in an encoder similar to that in FIG. 2 but with between nine and 11 segments, an extra transducer placed on the segment labeled 8 or the segment labeled 4 will give the proper number of unique codes. By way of example, Table II illustrates the codes for a nine segment encoder with the three transducers 92b-96b at segment 0 and transducer 98 at segment 8.

Operation of the printing device of FIG. 1 will be best understood by referring also to FIG. 3 which illustrates only a small portion of the print wheel 22 and the end portion 58 of cam member 56. Only the portions in the vicinity of print station (illustrated in phantom) are shown. As best seen in FIG. 30, the end 58 of the cam member is cam or wedged shaped forming an acute angle Q5 with the path taken by projecting portions 28 which may be preferably between 20 and 40 so that the projecting portions 28 do not tend to catch on the cam member end 58. As motor 32 continuously drives print wheel 22 in a clockwise direction as illustrated by arrow 101 in FIG. 1, encoder 34 continuously produces signals indicative of the characters as they approach print station 100. Assuming that it is desired to print the character a code corresponding to the letter C will be provided to comparator 40 on lines 42. Therefore, when encoder 34 produces signals indicating that the letter C is next approaching print station 100, the comparator senses that the code on cable 36 is equal to that on cable 42 and sends a signal to one shot 44.

This signal produces a momentary pulse which is amplified by amplifier 46 to activate actuator 50. When actuator 50 is energized, cam member 56 moves in the direction indicated by arrow 102. As illustrated in FIG. 3a, movement of cam member 56 may actually commence before character B has passed the cam member 56, because of the mechanical lag in the movement of the cam member. However, the cam member does not reach the path of movement of the characters until after the character B has passed the cam member as illustrated in FIG. 3b. Therefore, the letter C is considered next approaching when a comparison signal is generated. As illustrated in FIGS. 3c and 3d, when the rearwardly projecting stem 28 of the symbol element for the letter C comes into contact with cam member 56, the symbol element for C is propelled in the direction of arrow 104. The mass of the symbol element 24, stem 28 and resilient finger 26 must be sufficiently great to provide the required momentum to cause the character to strike the recording medium 14 (really the ribbon 16). That is, unlike the conventional printer in which the actuator actually drives the character and recording medium into contact, in this printer the character is carried by momentum into the recording medium. Therefore, the spring constant of elements 26, FIG. 1, must be sufficiently low to permit this action to occur.

An important feature of the present invention is the self-return of the cam element 56, shown in FIG. 3d. The movement of the symbol element for character C in the direction of arrow 104 causes an equal and opposite force to be applied to cam member 56. By the time this force occurs, the momentary signal applied to actuator 50 is terminated and the magnetic field, due to this signal, is sufficiently dissipated to permit movement of member 56 in the direction shown by arrow 106. The counterforce exerted by the stem 28 on the cam surface is sufficient to drive the cam element 56 out of the path of travel of the projecting stems 28 before the next character, D, arrives at the cam member. It should be obvious that the cam member need not fully retract so that stop 62 is against the solenoid before the character D arrives, only that the cam member be out of the path of stems 28 (the socalled second position in the claims that follow).

The relative mass of cam member 56 and character 24 plus stem 28 must be of the same order of magnitude though not necessarily equal. If the mass of ele ments 24 and 28 is much greater than the mass of member 56, the latter will bounce off and the character wont move, and vice versa. The relative masses are not critical however.

From the description of the operation of the printer, it will be noted that the timing of insertion of cam member 56 into the path of movement of the print wheel 22 has to be reasonably good. However, the return of the cam element to its rest position is completely asynchronous. With proper design of the respective masses of the cam element and elements 24, 26, and 28, the print wheel itself controls the return of the cam element to its initial position. There is no need for special retraction circuits as in the Sariti patent, for example. Also, when the character contacts the ribbon, because no hammer is forcing it against the ribbon, it will be driven away from the ribbon and no smearing of the radially moving character will occur.

From FIG. 1 it is seen that the tension spring 64 tends to pull the cam member 56 away from the path of movement of the print wheel. This spring is of very light spring constant, however, and has no appreciable effect on moving the cam member in a time frame of movementof one character past the cam member. The main purpose of the spring is merely to hold the cam member in its retracted position when no printing is desired.

The print wheel 22 may take many forms other than the one illustrated. For example, as illustrated in FIG. 4, the print wheel may be solid, that is the spaces between spring elements 24 may be filled in. Then each character may be slidably attached to the rotating member 22 and spring retained by compression springs 29 in the position shown for the character B. In that case, only the character and projecting portion move (see character C) in the direction of arrow 104 while the remainder of the print wheel remains axially stationary. Also, the various print characters may be mounted on a chain-like arrangement which moves them linearly past the print station. Here again the characters are slidably attached to the chain and held by means of suitable spring away from the recording medium except when they are in the path of cam member 56. Particularly in the latter described printer there may be an actuator for each print column forming a printed line. Then some linkage may be required be tween the actuator and the portion 58 of the cam member which actually contacts the print characters. All of these arrangements are considered to be within the scope of the invention as set forth in the following claims. The disc system illustrated was designed to 0p erate at a rate of approximately 3,000 characters per second past print station 100, allowing a printing rate of 30 characters per second. This goal was achieved in an actual apparatus. In fact, near perfect registration oecurred'at speeds several times that fast utilizing equipment designed for the slower speed.

What is claimed is:

1. Apparatus for printing on a recording medium comprising, in combination:

a rotatable member including, at a plurality of spaced points along the periphery thereof, symbol elements each movable into contact with said recording medium but normally biased away therefrom,

each having a projecting portion opposite the symbol shape on said symbol element;

drive means for rotating said rotatable member for moving said symbol elements seriatim past a print station;

actuator means at said print station including a cam member movable between a first position interposed in the path occupied by said projecting portions as said symbol elements are rotated and a second normal position away from said path, said actuator being responsive to a momentary signal applied thereto for causing said cam member to be moved to said first position, the surface of said cam member when at said first position facing said projecting portions and forming an acute angle with the plane of the path taken by said symbol elements, said cam member when in said first position being arranged so that the projecting portion of the next symbol element of said plurality of symbol elements to come'in contact therewith will thereby receive a force causing said element to be moved into contact with said recording medium, an oppositely directed force being thereby applied to said cam member; and

means responsive to a desired one of said symbol elements being next to approach said print station for producing said momentary signal for moving said cam member to said first position, said momentary signal terminating before said desired symbol element is at said print station, said oppositely directed force causing said cam member to be moved to said second position concurrently with the movement of said next symbol element.

2. The combination as set forth in claim 1 wherein said rotatable member comprises a rotatable disc and a plurality of flexible elongated elements extending radially therefrom, the free end of each including a symbol element, arranged to be rotated proximate to but normally separated from said recording medium.

3. Apparatus for printing on a recording medium comprising, in combination:

a moving element having a plurality of symbol elements coupled thereto for moving seriatim past a print station, each element being movable at said print station toward said recording medium; and

actuator means including a cam member movable be tween a first position interposed in the path taken by said symbol elements as they are moved past said print station and a second position away from said path, responsive to a source of signals indicative of the approach of a desired one of said symbol elements to said print station for applying a force to said cam member to cause it to be moved to said first position, said force terminating before said desired symbol element is at said print station, the contact of said desired symbol element and cam surface creating opposing forces on the two elements for respectively causing said desired symbol element to be moved into contact with said recording medium and concurrently causing said cam member to be moved to said second position.

4. Apparatus for printing on a recording medium comprising, in combination:

a plurality of symbol elements movable seriatim past actuator, for causing said cam member to be moved to said first position and being arranged to cording medium to cause said next element to be moved into contact with said recording medium, an oppositely directed force being thereby applied to said cam member; and

actuator means at said print station including a cam means responsive to a desired one of said symbol elemember movable between two positions the first ments being next to approach said print station for with its cam surface positioned in the path taken by producing said momentary signal for moving said said symbol elements as they are moved past said cam member to said first position, said momentary print station and the second out of said path, but signal terminating before said desired symbol eleresponsive to a momentary signal applied to said ment is at said print station, said oppositely directed force causing said cam member to be moved to said second position.

contact the symbol element of said plurality of symbol elements which is next at said print station after said cam member is moved to said first posi- 6. Apparatus for printing on a recording medium comprising, in combination:

a rotatable member comprising a disc and a plurality tion, said contact causing a force to be exerted on said next symbol element to move it into contact with said recording medium, said force from said contact concurrently causing said cam member to of symbol elements each extending through a respective one of a plurality of spaced apart openings in said disc near the periphery thereof with each of said openings through said member being parallel be moved to said second position; and to the axis of rotation of said member so that the means responsive to a desired one of said symbol elesymbol portion of each element is on one side of ments being next to approach said print station for the disc, the symbols being arranged to pass serially producing said signal for moving said cam member through a print station, each symbol element havto said first position, said signal terminating before ing on the other side of said disc a projecting porsaid desired symbol element is at said print station. tion, said symbol elements being biased away from 5. Apparatus for printing on a recording medium but movable to said recording medium at said print comprising, in combination: station;

a rotatable member including a disc and a plurality drive means for rotating said rotatable member for of symbol elements extending through aplurality of moving said symbol elements seriatim past said openings in said disc near the periphery thereof print station; and parallel to the axis of rotation thereof so that actuator means at said print station including a cam the symbol portion of each element is on one side member movable between a first position interof the disc arranged to pass serially through a print posed in the path occupied by said projecting porstation and a projecting portion of each element is tions as said symbol elements are rotated and a secon the other side thereof, said symbol elements ond normal position away from said path, said acbeing biased away from but movable through said tuator being responsive to a momentary signal apopenings to said recording medium; plied thereto for causing said cam member to be drive means for rotating said rotatable member for moved to said first position the surface of said cam moving said symbol elements seriatim past said member when at said first position facing said proprint station, the axis of rotation being substantially jecting portions and forming an acute angle with normal to said recording medium at said print stathe plane of the path taken by said symbol eletion; ments, said cam member when in said first position actuator means at said print station including a cam being arranged so that the projecting portion of the member movable, along a line generally parallel to next symbol element of said plurality of symbol elesaid axis of rotation, between a first position interments to come in contact therewith will thereby reposed in the path occupied by said projecting porceive a force causing said element to be moved into tions as said symbol elements are rotated, and a contact with said recording medium, an oppositely second position away from said path, said actuator directed force being thereby applied to said cam being responsive to a momentary signal applied member; and thereto for causing said cam member to be moved means responsive to a desired one of said symbol eleto said first position, the surface of said cam memments being next to approach said print station for her, when at said first position, facing said projectproducing said momentary signal for moving said ing portions and forming an acute angle with the cam member to said first position, said momentary plane of the path taken by said symbol elements, signal terminating before said desired symbol elesaid cam member when in said first position being ment is at said print station, said oppositely diarranged so that the projecting portion of the next rected force causing said cam member to be moved symbol element to come in contact therewith will to said second position. thereby receive a force directed toward said re- 

1. Apparatus for printing on a recording medium comprising, in combination: a rotatable member including, at a plurality of spaced points along the periphery thereof, symbol elements each movable into contact with said recording medium but normally biased away therefrom, each having a projecting portion opposite the symbol shape on said symbol element; drive means for rotating said rotatable member for moving said symbol elements seriatim past a print station; actuator means at said print station including a cam member movable between a first position interposed in the path occupied by said projecting portions as said symbol elements are rotated and a second normal position away from said path, said actuator being responsive to a momentary signal applied thereto for causing said cam member to be moved to said first position, the surface of said cam member when at said first position facing said projecting portions and forming an acute angle with the plane of the path taken by said symbol elements, said cam member when in said first position being arranged so that the projecting portion of the next symbol element of said plurality of symbol elements to come in contact therewith will thereby receive a force causing said element to be moved into contact with said recording medium, an oppositely directed force being thereby applied to said cam member; and means responsive to a desired one of said symbol elements being next to approach said print station for producing said momentary signal for moving said cam member to said first position, said momentary signal terminating before said desired symbol element is at said print station, said oppositely directed force causing said cam member to be moved to said second position concurrently with the movement of said next symbol element.
 2. The combination as set forth in claim 1 wherein said rotatable member comprises a rotatable disc and a plurality of flexible elongated elements extending radially therefrom, the free end of each including a symbol element, arranged to be rotated proximate to but normally separated from said recording medium.
 3. Apparatus for printing on a recording medium comprising, in combination: a moving element having a plurality of symbol elements coupled thereto for moving seriatim past a print station, each element being movable at said print station toward said recording medium; and actuator means including a cam member movable between a first position interposed in the path taken by said symbol elements as they are moved past said print station and a second position away from said path, responsive to a source of signals indicative of the approach of a desired one of said symbol elements to said print station for applying a force to said cam member to cause it to be moved to said first position, said force terminating before said desired symbol element is at said print station, the contact of said desired symbol element and cam surface creating opposing forces on the two elements for respectively causing said desired symbol element to be moved into contact with said recording medium and concurrently causing said cam member to be moved to said second position.
 4. Apparatus for printing on a recording medium comprising, in combination: a plurality of symbol elements movable seriatim past a print station and normally biased away from said recording medium at least at said print station but movable into contact therewith: actuator means at said print station including a cam member movable between two positions the first with its cam surface positioned in the path taken by said symbol elements as they are moved past said print station and the second out of said path, but responsive to a momentary signal applied to said actuator, for causing said cam member to be moved to said first position and being arranged to contact the symbol element of said plurality of symbol elements which is next at said print station after said cam member is moved to said first position, said contact causing a force to be exerted on said next symbol element to move it into contact with said recording medium, said force from said contact concurrently causing said cam member to be moved to said second position; and means responsive to a desired one of said symbol elements being next to approach said print station for producing said signal for moving said cam member to said first position, said signal terminating before said desired symbol element is at said print station.
 5. Apparatus for printing on a recording medium comprising, in combination: a rotatable member including a disc and a plurality of symbol elements extending through a plurality of openings in said disc near the periphery thereof and parallel to the axis of rotation thereof so that the symbol portion of each element is on one side of the disc arranged to pass serially through a print station and a projecting portion of each element is on the other side thereof, said symbol elements being biased away from but movable through said openings to said recording medium; drive means for rotating said rotatable member for moving said symbol elements seriatim past said print station, the axis of rotation being substantially normal to said recording medium at said print station; actuator means at said print station including a cam member movable, along a line generally parallel to said axis of rotation, between a first position interposed in the path occupied by said projecting portions as said symbol elements are rotated, and a second position away from said path, said actuator being responsive to a momentary signal applied thereto for causing said cam member to be moved to said first position, the surface of said cam member, when at said first position, facing said projecting portions and forming an acute angle with the plane of the path taken by said symbol elements, said cam member when in said first position being arranged so that the projecting portion of the next symbol element to come in contact therewith will thereby receive a force directed toward said recording medium to cause said next element to be moved into contact with said recording medium, an oppositely directed force being thereby applied to said cam member; and means responsive to a desired one of said symbol elements being next to approach said print station for producing said momentary signal for moving said cam member to said first position, said momentary signal terminating before said desired symbol element is at said print station, said oppositely directed force causing said cam member to be moved to said second posItion.
 6. Apparatus for printing on a recording medium comprising, in combination: a rotatable member comprising a disc and a plurality of symbol elements each extending through a respective one of a plurality of spaced apart openings in said disc near the periphery thereof with each of said openings through said member being parallel to the axis of rotation of said member so that the symbol portion of each element is on one side of the disc, the symbols being arranged to pass serially through a print station, each symbol element having on the other side of said disc a projecting portion, said symbol elements being biased away from but movable to said recording medium at said print station; drive means for rotating said rotatable member for moving said symbol elements seriatim past said print station; actuator means at said print station including a cam member movable between a first position interposed in the path occupied by said projecting portions as said symbol elements are rotated and a second normal position away from said path, said actuator being responsive to a momentary signal applied thereto for causing said cam member to be moved to said first position the surface of said cam member when at said first position facing said projecting portions and forming an acute angle with the plane of the path taken by said symbol elements, said cam member when in said first position being arranged so that the projecting portion of the next symbol element of said plurality of symbol elements to come in contact therewith will thereby receive a force causing said element to be moved into contact with said recording medium, an oppositely directed force being thereby applied to said cam member; and means responsive to a desired one of said symbol elements being next to approach said print station for producing said momentary signal for moving said cam member to said first position, said momentary signal terminating before said desired symbol element is at said print station, said oppositely directed force causing said cam member to be moved to said second position. 